Coated paper possessing silky feel

ABSTRACT

The present invention relates to a coating composition for coated papers intended to be printed in offset, characterized in that the coating pigments are polyurethane microbeads having a size of less than 10 micrometers and advantageously of approximately 7 micrometers. It also relates to a paper having a silky feel, comprising at least one face coated with said coating composition and advantageously these two faces. This paper is also characterized in that it can be printed in offset with low, indeed even nonexistent, setting-off and chalking.

The present invention relates to a coating composition comprisingpolyurethane microbeads and to the paper coated on at least one facewith said coating composition, advantageously on both its faces.

Papers of various types or with various compositions can be coated so asto enhance the quality of their surface.

A coating composition used conventionally for this purpose is adispersion comprising pigments, which can be organic or inorganicpigments.

The coating composition can also comprise other additives.

Paper intended for printing, such as paper for books, magazines,brochures or annual reports, or paper intended for packaging is oftenmanufactured by coating a base paper using one or more coating units,sometimes followed by calendering.

The final product should generally have surface characteristics improvedfor the use which is made of it.

These surface characteristics are mainly reflected by a modification inthe visual appearance of the paper, i.e. by an increase in itswhiteness, in its gloss or in its opacity.

The search for a certain surface feel for the sheet covered with thecoating is fairly new per se.

In the prior patent FR 2 791 368, the Applicant Company had beeninterested in providing a packaging sheet which would include acharacteristic which would recall one of the essential characteristicsof the product to be packaged, in particular for cosmetic products to beapplied to the skin.

The Applicant Company thus proposed to provide a characteristic whichevokes the destination, namely the skin, that is to say the feel or thesoftness of the skin.

The desired feel could be obtained by applying, to the surface of thesheet, a layer which comprised expandable thermoplastic microspheres.

This patent was targeted, inter alia, at solving the problems ofmachinability of these sheets during printing, in particular theproblems of separation of sheets in stacks because of the antislipproperties of the added microspheres.

Generally, coated thin paper often raises serious problems duringprinting or during converting operations, the commonest being in thecontext of offset printing: chalking and setting-off.

Chalking corresponds to the detachment of the coating in the form of adry powder during printing, which phenomenon can become more marked withsuccessive passages of the sheet in printing or converting machines.

This phenomenon results from poor binder/pigment metering, the pigmentbeing in an excessively high percentage with respect to the binder.

It can also result from the size and the shape of the pigment particles.

Setting-off corresponds to the transfer of print from one sheet to theother, in particular during stacking.

More or less intense ink stains are thus often observed on the verso ofa sheet, originating from the ink of the recto of the sheet underneath.

When the stack is taken apart, the crunching of the sheets is heard,more or less loudly.

This phenomenon, if it is slight, is known as setting-off.

It is picking if paper elements participate in the separation.

It is stack adhesion if it is no longer possible to separate thesesheets from one another.

The paper can participate in this phenomenon due to the fact that apaper unsuited to the ink can have a poor ability to accept the ink andto the fact that a paper having an uneven surface condition requires toomuch ink for correct printing.

In the light of these considerations, the Applicant Company hasendeavored to provide a paper having a silky feel, which are coatedpreferably on these two faces and which does not exhibit the problems ofprintability and of machinability stated above.

A first aim of the invention is therefore to provide a coatingcomposition for coated papers intended to be printed in offset,characterized in that the pigments used in the coating are polyurethanemicrobeads having a mean size of less than 10 micrometers andadvantageously of approximately 7 micrometers.

In particular, the invention is characterized in that the binder used inthe coating is an aqueous emulsion of polyurethane and preferably a softpolyurethane having a degree of elongation of greater than or equal to500%.

In particular, the invention is characterized in that the level ofbinder with respect to the pigments is greater than or equal to 40% bydry weight and preferably greater than or equal to 200%.

Another aim of the invention is to provide a coated paper having a silkyfeel, comprising at least one face coated with a coating composition asdefined above and advantageously these two faces.

Said paper is characterized in that it can be printed in offset withlow, indeed even nonexistent, setting-off and chalking.

The paper is based on cellulose fibers and has a grammage of between 70and 500 g/m², preferably between 100 and 300 g/m².

According to a specific case, the paper is transparent or translucent.

More particularly, the paper constitutes a tracing paper.

The term “tracing paper” is understood to mean a paper as defined by theinternational standard ISO 4046-1978 in part 6.94.

More particularly, the invention is characterized in that said paper isa tracing paper, obtained in particular by hard beating of the cellulosefibers of which it is composed.

In particular, the invention is characterized in that the weight of saidlayer on at least one of the faces is between 2 and 10 g/m² on a drybasis, preferably between 3 and 6 g/m².

Said paper is also characterized by a coefficient of dynamic frictionbetween its coated recto face and its other verso face, also coated, ofless than or equal to 1.0, which measurement is carried out according tothe standard NF-Q-03-082 with a block weighing 200 g.

The coefficient of dynamic friction reflects the ability of the sheet tostick at the start of a displacement; the higher it is, the greater thedifficulty in initiating a movement of a sheet in a stack.

Comparative examples and nonlimiting examples of the implementation ofthe invention and other advantages, possibly, are described below.

EXAMPLES Examples 1 to 5

A composition comprising polyurethane microbeads of different sizes, asmentioned in table 1, is applied, using an air blade coater, to a sheetof base paper having a grammage of 100 g/m², more specifically thereference Conqueror CX 22 sold by the Applicant Company, in a proportionof 5 g/m² approximately per face.

The composition comprising polyurethane microbeads is prepared in anaqueous medium and comprises, as dry weight:

-   -   24.7 parts of water    -   0.1 part of wetting agent    -   33.3 parts of an aqueous polyurethane emulsion Witcobond 290 H    -   20 parts of polyurethane microbeads    -   0.5 part of pH regulator    -   1 part of thickener.

Examples 6 to 13

A composition comprising polyurethane microbeads with a mean size ofapproximately 7 micrometers is applied, using an air blade coater, to asheet of base paper having a grammage of 100 g/m², more specifically thereference Conqueror CX 22 sold by the Applicant Company, in a proportionof 5 g/m² on a dry basis approximately per face, the level of binderbeing varied with respect to the weight of pigment as mentioned in table2.

The composition comprising polyurethane microbeads is prepared in anaqueous medium and comprises, as dry weight:

-   -   variable parts on a dry basis of an aqueous polyurethane        emulsion Rolflex PAD    -   20 parts of polyurethane microbeads Daiplacoat RHU 5070    -   0.5 part of pH regulator    -   1 part of a thickener        Implementation of the Tests:

The feel of the face comprising the coating of polyurethane microbeadsis assessed by hand.

The response of the virgin paper to the chalking test is determined soas to demonstrate the propensity of the coating to disintegrate whenpassing through a machine. The chalking test is carried out in thefollowing way:

-   -   a) a block with dimensions of 40 mm×40 mm and with a weight of 2        kg, mounted on a trolley and covered with a black felt made of        fabric (mixture of 70% wool, 30% polyamide), is applied to one        or more paper sheets coated with said coating,    -   b) a single pass is carried out over the paper, so as to cover a        total length of 6 m,    -   c) the felt is removed from the block,    -   d) the reflectivity of the felt is measured using an Elrepho        device (the value measured being approximately 1.4 for the        untreated fabric),

The response of the printed paper to the setting-off test is determined,so as to demonstrate the propensity of the paper to result insetting-off during machine stacking. The setting-off test is carried outin the following way:

-   -   a) test specimens of the paper to be tested are prepared with a        width of 48 mm and a length of 250 mm,    -   b) a Prufbau test device having a station No. 1 for printing and        a station No. 2 for print transfer is used,    -   c) the pressure of the station No. 1 is adjusted to 1 000 N and        the pressure of the station No. 2 is adjusted to 400 N,    -   d) the speed of the device is adjusted to 0.5 m/sec,    -   e) the inking roller of the station No. 1 is inked for 30 sec        with a blue ink of Huber 408010 type,    -   f) a conveyor equipped with a test specimen is positioned in        front of the station No. 1,    -   g) an uninked roll is positioned on the station No. 2,    -   h) the test specimen is printed on the station No. 1,    -   i) the stopwatch is started immediately after printing,    -   j) a test specimen of the same paper is positioned, using an        adhesive tape, on the blank roll of the station No. 2,    -   k) once 120 sec have passed on the stopwatch, the conveyor        equipped with the printed test specimen is set underway as far        as the blanket,    -   l) the test specimen of the station No. 2 is immediately        separated from its roll,    -   m) the optical density of the cyan transferred to the blank test        specimen is measured using an Xrite densitometer.        Results:

The results of the tests for examples 1 to 5 are presented in table 1.

It is found that, even though the majority of the papers have a fairlysatisfactory response to the setting-off and chalking tests, onlymicrobead sizes of less than 10 micrometers make it possible to obtainan acceptable silky feel.

The results of the tests for examples 6 to 13 allow it to be concludedthat, on choosing a size of microbeads which makes it possible to obtainthe desired silky feel, it is subsequently necessary to choose a levelof binder of greater than or equal to 40% in order to have correctchalking and of greater than or equal to 200% in order to have correctsetting-off.

This is because chalking is considered to be satisfactory below 2 andsetting-off below 0.1.

A scanning electron microscope view of a paper exhibiting, from theviewpoint of the Applicant Company, a silky feel has been represented inFIGS. 1 and 2.

This paper was obtained by depositing, on a Conqueror CX 22 base paperhaving a grammage of 100 g/m², the following coating composition onthese two faces:

-   -   2.8 kg of water,    -   0.03 kg of wetting agent Texapon Z95P,    -   81 kg of binder Rolflex PAD sold by Lamberti,    -   6.1 kg of polyurethane microbeads Daiplacoat RHU 5070 sold by        Promecome,    -   1 kg of a thickener CMC Blanose 7M65, at 2.5%, sold by Aqualon,        in a proportion of 5.4 g/m² as dry weight per face, the amounts        being expressed as commercial weight.

The paper obtained was subsequently calendered.

The photographs respectively represent a top view, magnified 200 times,and a cutaway view, magnified 2 000 times.

The paper obtained really exhibits a silky feel on these two faces.

On measuring the coefficient of dynamic friction of the coated paper onitself according to the standard NF-Q-03-082 with a block weighing 200g, a value of 0.8 is obtained, which value is able to ensure goodmachinability of the sheets on printing or converting machines. TABLENO. °1 Name of the particles Daiplacoat Decolam F7 Decolam FT Decolam FDecolam W Size of the 7 10 28 28 50 particles (in micrometers) Silkyfeel YES Passable Moderate Moderate NO Chalking 1.8 1.6 1.6 1.6 1.4Setting-off 0.4 0.32 0.41 0.39 0.2

TABLE NO. 2 Level of binder % pigment 10% 20% 30% 40% 50% 100% 150% 400%Silky feel YES YES YES YES YES YES YES YES Chalking 15.5 6 3.6 1.9 1.71.5 1.5 1.5 Setting-off 0.282 0.326 0.327 0.354 0.367 0.123 0.104 0.07

1. A coating composition for coated papers intended to be printed inoffset comprising pigments and a binder, wherein the coating pigmentsare polyurethane microbeads having a size of less than 10 micrometers.2. The coating composition as claimed in claim 1, wherein the binderused is an aqueous emulsion of polyurethane.
 3. The coating compositionas claimed in claim 2, wherein the level of binder with respect to thepigments is greater than or equal to 40% by weight.
 4. A paper having asilky feel, comprising at least one face coated with a coatingcomposition as claimed in claim
 1. 5. The paper as claimed in claim 4,wherein the coefficient of dynamic friction between the coated rectoface and the other verso face, also coated, of this paper is less thanor equal to 1.0, which measurement is carried out according to thestandard NF-Q-03-082 with a block weighing 200 g.
 6. The paper asclaimed in claim 4, wherein the weight of the coating applied to atleast one face is between 2 and 10 g/m² by dry weight.
 7. The paper asclaimed in claim 4, wherein it constitutes a tracing paper, obtained inparticular by hard beating of the cellulose fibers of which it iscomposed.
 8. The paper as claimed in claim 1, wherein the polyurethanemicrobeads have a size of approximately 7 micrometers.
 9. The coatingcomposition as claimed in claim 2, wherein the binder has a degree ofelongation of greater than or equal to 500%.
 10. The coating compositionas claimed in claim 3, wherein the level of binder with respect to thepigments is greater than or equal to 200%.
 11. The paper as claimed inclaim 4, wherein the two faces are coated with the coating composition.12. The paper as claimed in claim 6, wherein the weight of the coatingapplied to the at least one face is between 3 and 6 g/m².
 13. The paperas claimed in claim 5, wherein the weight of the coating applied to atleast one face is between 2 and 10 g/m² by dry weight.
 14. A paperhaving a silky feel, comprising at least one face coated with a coatingcomposition as claimed in claim
 2. 15. The paper as claimed in claim 14,wherein the coefficient of dynamic friction between the coated rectoface and the other verso face, also coated, of this paper is less thanor equal to 1.0, which measurement is carried out according to thestandard NF-Q-03-082 with a block weighing 200 g.
 16. The paper asclaimed in claim 14, wherein the weight of the coating applied to atleast one face is between 2 and 10 g/m² by dry weight.
 17. The paper asclaimed in claim 14, wherein it constitutes a tracing paper, obtained inparticular by hard beating of the cellulose fibers of which it iscomposed.
 18. A paper having a silky feel, comprising at least one facecoated with a coating composition as claimed in claim
 3. 19. The paperas claimed in claim 18, wherein the coefficient of dynamic frictionbetween the coated recto face and the other verso face, also coated, ofthis paper is less than or equal to 1.0, which measurement is carriedout according to the standard NF-Q-03-082 with a block weighing 200 g.20. The paper as claimed in claim 18, wherein the weight of the coatingapplied to at least one face is between 2 and 10 g/m² by dry weight.